The present invention generally relates to a high temperature superconductor ("HTSC") device. More particularly, the invention relates to structural geometries for rotational HTSC devices to achieve lower rotational dissipations, higher magnetic pressures and higher rotational velocities.
Efficient electrical energy storage is useful in numerous practical applications. Diurnal storage of electricity is important to electric utilities in order to efficiently utilize base load generating plants and to meet the varying load demands of their customers. Base load plants can charge storage units at night when demand is low, and then peak demands can be met by discharging the storage units during the peak hours.
Energy storage can also play a substantial role in eliminating or postponing the installation of larger capacity power lines. For example, power can be transferred from a baseload generating plant to a substation having energy storage units at night when demand is low. During peak power demand times, the energy storage units can be discharged. These energy storage units can also be located in other parts of the electrical distribution system: utility parks where large amounts of energy can be stored; in tandem with photovoltaic or wind energy generation facilities that are time dependent; substation units; and individual companies and homes. Similar energy storage units can be used on electric vehicles such as cars and buses, or as wayside energy storage for electric trains.
Flywheels are often considered for energy storage unit applications. Their primary advantages are modularity, high energy storage density (Wh/kg), and high efficiency input and output of electrical energy. The ability to produce high strength flywheel rotors and the ability to efficiently transfer energy in and out of a flywheel are well known and will not need to be discussed in this application.
The primary disadvantage of conventional flywheels is inefficiency in the standby mode. Substantial energy losses occur because the conventional ball bearings that support the flywheel structure have high losses. Conventional ball bearings have relatively large coefficients of friction, are subject to wear and also require lubrication.
Alternatively, magnetic bearings can be used to support the flywheel structure. Conventional magnetic bearings have no contacting parts, require no lubrication, and often have lower losses than ball bearings. However, magnetic bearings require position sensors and feedback electronics to keep the bearings stable. Further, energy losses associated with conventional magnetic bearings are still sufficiently high that diurnal storage of energy in a flywheel is relatively inefficient. Energy losses in a conventional magnetic bearing are attributable to magnetic drag and parasitic losses from passing current through the windings of the electromagnets in the bearings. A state-of-the-art flywheel energy storage unit with a magnetic bearing typically loses about 1% of the stored energy per hour due to energy losses attributable to the bearings.
Superconducting bearings have the potential to reduce such energy loss to very low values. One example of such a structure is described in U.S. patent application Ser. No. 07/736,677, now U.S. Pat. No. 5,214,981 which is incorporated by reference herein in its entirety, and is assigned to the owner of the instant invention. In this other application, a permanent magnet is rotated over an HTSC material structure. The disadvantage of this configuration is that any azimuthal nonuniformity in the rotating permanent magnets produces an alternating current magnetic field at the surface of the HTSC material. This field, in turn, induces hysteresis losses in the HTSC material; and the associated energy must be removed at cryogenic temperatures. Another disadvantage of this type of bearing is that the permanent magnet has a relatively low tensile strength and therefore cannot withstand high rotational speeds. In addition, there is a limitation in magnetic pressure due to the magnetic fields of the permanent magnets.
It is therefore an object of the invention to provide a novel low-loss magnetic bearing and method of use in a high-efficiency flywheel energy storage device.
It is a further object of the invention to provide an improved flywheel with low standby energy losses when the storage time is about one day or longer.
It is yet another object of the invention to provide a novel low drag force magnetic bearing comprising a permanent magnet, an HTSC material and a ferromagnetic rotor.
It is still a further object of the invention to provide an improved rotor capable of very high rotational speeds using a superconductor material bearing.
It is an additional object of the invention to provide a novel high-efficiency flywheel energy storage device with a high degree of rotational symmetry.
It is yet a further object of the invention to provide an improved high-efficiency flywheel energy storage device with high magnetic field azimuthal homogeneity.
It is still an additional object of the invention to provide a novel high-efficiency flywheel energy storage device having lower rotational dissipation and higher magnetic levitation pressure.
It is yet another object of the invention to provide an improved flywheel energy storage device which minimizes hysteresis and eddy current losses.
Other objects, features and advantages of the present invention will be readily apparent from the following description of the preferred embodiments thereof, taken in conjunction with the accompanying drawings described below wherein like elements have like numerals throughout the several views.